ruby: how to fire and forget a subprocess?

Question

Hi,

I have a long running process and I need it to launch another process (that will run for a good while too). I need to only start it, and then completely forget about it.

I managed to do what I needed by scooping some code from the Programming Ruby book, but I'd like to find the best/right way, and understand what is going on. Here's what I got initially:

exec("whatever --take-very-long") if fork.nil?
Process.detach($$)

So, is this the way, or how else should I do it?

After checking the answers below I ended up with this code, which seems to make more sense:

(pid = fork) ? Process.detach(pid) : exec("foo")

I'd appreciate some explanation on how fork works. [got that already]

Was detaching $$ right? I don't know why this works, and I'd really love to have a better grasp of the situation.

Answer 1

The fork function separates your process in two.

Both processes then receive the result of the function. The child receives a value of zero/nil (and hence knows that it's the child) and the parent receives the PID of the child.

Hence:

exec("something") if fork.nil?

will make the child process start "something", and the parent process will carry on with where it was.

Note that exec() replaces the current process with "something", so the child process will never execute any subsequent Ruby code.

The call to Process.detach() looks like it might be incorrect. I would have expected it to have the child's PID in it, but if I read your code right it's actually detaching the parent process.

Answer 2

Alnitak is right. Here's a more explicit way to write it, without $$

pid = Process.fork
if pid.nil? then
  # In child
  exec "whatever --take-very-long"
else
  # In parent
  Process.detach(pid)
end

The purpose of detach is just to say, "I don't care when the child terminates" to avoid zombie processes.

Answer 3

Detaching $$ wasn't right. From p. 348 of the Pickaxe (2nd Ed):

$$ Fixnum The process number of the program being executed. [r/o]

This section, "Variables and Constants" in the "Ruby Language" chapter, is very handy for decoding various ruby short $ constants - however the online edition (the first

So what you were actually doing was detaching the program from itself, not from its child. Like others have said, the proper way to detach from the child is to use the child's pid returned from fork().